serial: qcom-geni: Fix RTS behavior with flow control

When userspace enables flow control (CRTSCTS), the driver
deasserts RTS even when the receive buffer has space. This prevents the
peer device from transmitting, causing communication to stall.

The root cause is that the driver unconditionally uses manual RTS control
regardless of flow control mode. When CRTSCTS is set, the hardware should
automatically manage RTS based on buffer status, but the driver overrides
this by setting manual control.

Fix this by introducing port->manual_flow flag. In set_termios(), disable
manual flow when CRTSCTS is set. In set_mctrl(), only assert
SE_UART_MANUAL_RFR when manual_flow is active. Verified by enabling and
disabling hardware flow control with stty.

Signed-off-by: Anup Kulkarni <anup.kulkarni@oss.qualcomm.com>
Link: https://patch.msgid.link/20260310104155.339010-1-anup.kulkarni@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Anup Kulkarni

drivers/tty/serial/qcom_geni_serial.c

@@ -146,6 +146,7 @@ struct qcom_geni_serial_port {
 	int wakeup_irq;
 	bool rx_tx_swap;
 	bool cts_rts_swap;
+	bool manual_flow;
 
 	struct qcom_geni_private_data private_data;
 	const struct qcom_geni_device_data *dev_data;
@@ -250,7 +251,7 @@ static void qcom_geni_serial_set_mctrl(struct uart_port *uport,
 	if (mctrl & TIOCM_LOOP)
 		port->loopback = RX_TX_CTS_RTS_SORTED;
 
-	if (!(mctrl & TIOCM_RTS) && !uport->suspended)
+	if (port->manual_flow && !(mctrl & TIOCM_RTS) && !uport->suspended)
 		uart_manual_rfr = UART_MANUAL_RFR_EN | UART_RFR_NOT_READY;
 	writel(uart_manual_rfr, uport->membase + SE_UART_MANUAL_RFR);
 }
@@ -1401,11 +1402,21 @@ static void qcom_geni_serial_set_termios(struct uart_port *uport,
 	else
 		stop_bit_len = TX_STOP_BIT_LEN_1;
 
-	/* flow control, clear the CTS_MASK bit if using flow control. */
-	if (termios->c_cflag & CRTSCTS)
+	/* Configure flow control based on CRTSCTS flag.
+	 * When CRTSCTS is set, use HW/auto flow control mode, where HW
+	 * controls the RTS/CTS pin based FIFO state.
+	 * When CRTSCTS is clear, the CTS pin value is ignored for TX
+	 * path and RTS pin can be set/cleared using registers, for RX
+	 * path.
+	 */
+
+	if (termios->c_cflag & CRTSCTS) {
 		tx_trans_cfg &= ~UART_CTS_MASK;
-	else
+		port->manual_flow = false;
+	} else {
 		tx_trans_cfg |= UART_CTS_MASK;
+		port->manual_flow = true;
+	}
 
 	if (baud) {
 		uart_update_timeout(uport, termios->c_cflag, baud);